1. Field of the Invention
The present invention relates to a process for the continuous production of an acrylonitrile polymer melt. More specifically, the invention is concerned with a process for producing an acrylonitrile polymer in a substantially melted state by polymerizing acrylonitrile alone or a monomer mixture containing acrylonitrile, in the presence of water, at elevated temperature under increased pressure, wherein the polymerization is carried out using a persulfate as the polymerization initiator and under a particular forced stirring as defined in the specification, with the result that a runaway polymerization reaction is effectively suppressed, the melting of the resulting polymer is facilitated, and in addition a melt of an acrylonitrile polymer having a moderate molecular weight and able to be dyed can be produced in an industrially advantageous manner.
2. Description of the Prior Art
In recent years, it has been revealed that, when an acrylonitrile polymer is treated at high temperature under increased pressure in the presence of a small amount of a non-solvent, the polymer and the non-solvent form a homogeneous fluid which represents a fluidity like that of the melt of polyesters or polyamides. As regards processes for producing fibers by spinning such a melt, several proposals have been made as seen in U.S. Pat. No. 3,388,202, Disclosed Japanese Patent Applications Nos. 28982/1973, 49839/1973, 52832/1973, etc. In particular, most of these processes use water as the non-solvent for acrylonitrile polymers, and it is a remarkable feature of such processes that acrylic synthetic fibers are obtained without using conventional expensive solvents such as dimethylformamide, dimethyl sulfoxide, aqueous thiocyanate solutions, concentrated nitric acid, etc.
By applying this melting phenomenon to a polymerization system, there has been proposed, as Disclosed Japanese Patent Application No. 97683/1975, a process for the production of an acrylonitrile polymer melt simultaneously with the polymerization of acrylonitrile, and since then the use of such acrylonitrile polymer melts in the production of fibers, etc. is receiving attention from the viewpoint of process contraction (simplification), cost reduction, material economy, prevention of environmental pollution, etc.
In all these processes, however, a high-temperature, pressurized conditions exceeding one hundred and several tons of degrees are employed to melt acrylonitrile polymers in the presence of water. Accordingly, these processes involve various problems awaiting solution in the quality of the resulting polymer and in industrial operations, including discoloration of the polymer, danger due to high-temperature operations, increased energy cost due to high-temperature maintenance, ensurance of safety, reduction in cost, etc.
Especially, in such a process as the above-mentioned patent in which an acrylonitrile polymer melt is produced simultaneously with the polymerization of acrylonitrile, monomer is present at high concentration in the polymerization system and the polymerization system becomes highly viscous, so that an abnormal temperature rises due to a runaway of the polymerization reaction and local accumulation of heat due to a drop in heat transfer are caused, by which the discoloration of the resulting melt becomes more remarkable. With the enlargement of the scale of the process, such a runaway of the polymerization reaction and accumulation of heat causes more and more knotty problems, not only aggravating the discoloration of the polymer but also widening the molecular weight distribution of the polymer, and moreover affording even a possibility of creating a dangerous condition of causing the explosion of the polymerization tank by extraordinary pressure elevation.